Decades after the Nyanza Color and Chemical Co. plant in Ashland closed, the cleanup continues — and likely will for years to come.

From 1917 to 1978, harmful waste from textile dyes were released into streams and the Sudbury River.

Since 1982, Nyanza has been listed as a Superfund site through the U.S. Environmental Protection Agency. A routine five-year review of the cleanup and remediation work at the 35-acre site off Megunko Road was completed in May.

"We still have years to go, but hopefully not decades," said Dan Keefe, an EPA remedial project manager. "Since the site was listed in 1982, there has been significant progress in reducing human health and environmental risks."

In Massachusetts, there are 36 sites on the National Priority List (NPL) for cleanup of toxic materials and reuse of the property. Besides Nyanza, other Superfund sites in MetroWest include Natick Labs and Hocomomco Pond in Westborough.

The contamination from the Nyanza plant has been linked to a high number of cases of rare cancer, including eight deaths. Concerns are still being raised about the risks to people eating mercury-contaminated fish caught in the Sudbury River, where warning signs are posted along the banks.

Keefe said the cleanup has been split into four stages over the years with two considered complete, including the capping of a landfill and removing contaminated sediment from adjacent wetlands. In certain areas, vapor mitigation systems have been installed in homes to monitor the air quality.

The EPA plans to dump 140,000 tons of sand near the Reservoir No. 2 dam in Framingham to cap the contamination in the sediment, a plan opposed by a citizens group called Save the Sudbury River.

The proposed "thin-layer sand cap" — adding 6 inches of sand to the bottom of the river — will push the mercury down at a rate that would naturally take 100 to 200 years to complete, Keefe said. The EPA is waiting for funds to start the $11 million project.

Opponents claim the plan is based on "flawed science" and will destroy the ecosystem of the river.

"Mother Nature is taking care of the problem," said Laurene Hunt, a group organizer. "This is a waste of money and is not solving the problem."

Hocomomco Pond, a 23-acre water body off Otis Street in Westborough, was placed in the National Priority List in 1983. From 1928 until 1946 the site was used as a wood-treating and preservation operation, where creosote and waste were discharged into an unlined pit, according to the EPA website.

The site, owned by the town, is undergoing its five-year review along with 27 other Superfund sites across New England. A public meeting is scheduled at 7 p.m. Thursday in the Forbes Building, 45 W. Main St., offering an update on the project and possible reuse.

"The town is in the process of figuring out how to best use the land in the future," said Steven Baccari, Westborough’s director of public health.

Page 2 of 2 - The Natick Soldier Systems Center, commonly known as Natick Labs, a 74-acre facility situated on the eastern shore of Lake Cochituate, was built by the Army in 1954 and has since used the area for industrial, laboratory and storage. It became a Superfund site in 1994 after groundwater and air samples revealed high levels of heavy metals.

The EPA has determined the "site poses no immediate threat to human health or the environment while studies leading to site cleanup are being planned and conducted," according to its website.

It’s not all bad news. The former Sudbury Training Annex is now known as the Assabet River National Wildlife Refuge. The EPA removed the site from the National Priorities List in 2002.

The former Army annex located on 2,750 acres in portions of Maynard, Stow, Hudson, and Sudbury was established in 1942 during World War II. It served as an ammunition depot, test station, training and research area and a laboratory disposal area, according to the EPA.

"I still call it the ammo dump," said Maynard resident Paul Boothroyd, who grew up less than a mile away and is writing a book on the property.

Cleanup of the property began in 1985 when the Army and EPA removed more than 15,000 cubic yards of contaminated soil, 300 tanks and drums and capped a landfill that covered over 2-acres, according to the EPA. The landfill is still being monitored, Boothroyd said.

Jonathan Phelps can be reached at 508-626-4338 or jphelps@wickedlocal.com. Follow him on Twitter @JPhelps_MW.